The present invention relates generally to the field of semiconductor plasma processing systems such as photoresist ashers, and more specifically to a actively-cooled distribution plate for reducing reactive gas temperature for use in such systems.
In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate, such a silicon wafer. Typically, the substrate is coated with a photoresist, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. These remaining exposed portions are baked during a photostabilization process to enable the photoresist to withstand subsequent processing.
After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the baked photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is xe2x80x9cashedxe2x80x9d or xe2x80x9cburnedxe2x80x9d and the ashed or burned photoresist, along with the residue, is xe2x80x9cstrippedxe2x80x9d or xe2x80x9ccleanedxe2x80x9d from the surface of the substrate.
One manner of removing photoresist and residues is by rapidly heating the photoresist-covered substrate in a vacuum chamber to a preset temperature by infrared radiation, and directing a microwave-energized reactive plasma toward the heated substrate surface. In the resulting photoresist ashing process, wherein the reactive plasma reacts with the photoresist, the hot reactive gases in the plasma add heat to the surface of the substrate by means of convection. Heat energy on the order of 100 millliwatts per square centimeter (mW/cm2) is also added to the wafer as a result of the surface reaction. Excessive heat on the surface of the wafer can damage devices or portions thereof which have been formed on or in the wafer. In addition, excessive heat on the surface of the wafer can cause photoresist cracking during, for example, high-density ion implanted (HDII) wafer ash processes.
Reducing the temperature of the ashing process in the chamber will slow the reaction rate and thus the amount of heat added to the wafer by the surface reaction. However, the gas temperature, which is a function of the gas mixture and the applied microwave power, will remain unaffected by the reduced process temperature. The problem is exacerbated if the process includes a reaction catalyst such as carbon tetrafluoride (CF4) which tends to increase the rate of reaction due to increased production of atomic oxygen. As a result, the catalyst-assisted process results in higher temperature gases, even at lower process temperatures.
A typical plasma processing apparatus is shown in U.S. Pat. No. 5,449,410 to Chang et al. wherein an aluminum baffle plate or showerhead is provided for distributing gas into a plasma chamber. However, no means of controlling the temperature of the gas is shown. Accordingly, the apparatus shown will suffer from the adverse effects of high temperature gases as described above.
In addition, because individual wafers are processed in a serial fashion by known single-wafer process chambers, systems such as that shown in U.S. Pat. No. 5,449,410 exhibit a phenomenon known as the xe2x80x9cfirst wafer effectxe2x80x9d, which refers to secondary heating of subsequent wafers caused indirectly by the heating of the first-processed wafer. Specifically, upon completion of processing of the first wafer, the heated processed wafer and the process chamber side walls radiate heat toward the gas distribution baffle plate (typically made from quartz). The heated quartz plate then indirectly provides a secondary heating mechanism for subsequent wafers that are processed in the chamber. As a result, the first and subsequent wafers processed by the system exhibit process non-uniformities.
Still another problem with known baffle plates is that thermal gradients develop across the surface of the baffle plate. Because such baffle plates are typically made of quartz, due to their ability to withstand high process temperatures, they tend to exhibit poor thermal conductivity as well as undesirable infrared (IR) wavelength absorption characteristics. In addition, the temperature of a quartz baffle plate can be difficult to control if IR wavelength energy is absorbed from the wafer with no means for sinking or dissipating the absorbed radiant energy. As a result, process uniformity and system throughput are adversely affected.
Thus, it is an object of the present invention to provide a mechanism for reducing the temperature of gases used in a wafer processing system such as a photoresist asher to prevent damage to the wafer during the ashing process. It is a further object of the present invention to reduce the temperature of reactive gases required by low temperature processes, by incorporating cooling means into a gas distribution or baffle plate used therein. It is yet a further object of the invention to improve wafer-to-wafer process uniformity in such processes, by eliminating secondary heating caused by the xe2x80x9cfirst wafer effectxe2x80x9d. It is still a further object of the invention to provide a mechanism for providing a relatively flat temperature profile across the surface of the gas distribution or baffle plate, thereby improving both high and low temperature within-wafer process uniformity.
A plasma processing system is provided, having processor chamber walls and/or a gas distribution or baffle plate equipped with integral cooling passages for reducing an operating temperature thereof during processing of a wafer by the system. Cooling medium inlets and outlets are connected to the cooling passages to permit circulation of a cooling medium through the cooling passages. Preferably, the chamber walls and the gas distribution or baffle plate are comprised of aluminum and the cooling passages are machined directly therein. The cooling medium may be either liquid (e.g., water) or gas (e.g., helium or nitrogen).
The baffle plate comprises a generally planar, apertured, gas distribution central portion surrounded by a flange, into both of which the cooling passages may extend. The cooling passages in the chamber walls and those in the gas distribution or baffle plate may be in communication with one another so as to permit them to share a single coolant circulating system. Alternatively, the cooling passages in the chamber walls and those in the gas distribution or baffle plate may not be in communication with one another, so as to provide independent circulating systems (gas or liquid) for each, thereby enabling independent temperature control and individual flow control thereof. In operation, the cooling medium in the chamber wall cooling passages is maintained approximately within the range of 15xc2x0 C.-30xc2x0 C., and the cooling medium in the gas distribution or baffle plate cooling passages is maintained approximately within the range of 15xc2x0 C.-80xc2x0 C.